The present invention concerns a multiple pressure switch comprising a first and a second snap switch, which under spring prestress each assume a starting position and have a first and second point of application, respectively, for an actuating member, which is supported on a total of three application points lying on the corners of a conceived triangle, and with gradual increase of an external force acting on the actuating member within said triangle causes first the first and then the second snap switch to snap over into a final position.
In general, such multiple pressure switches have a housing, which is subdivided into two chambers by a membrane. A pressure to be monitored is effective on one side of the membrane. A pressure plate is arranged on the other side of the membrane, onto which the membrane transmits the pressure and which for its part is supported on said actuating member. The actuating member is generally designed as a three-armed lever and extends to one of the points of application with each of its three arms. The points of application lie on the corners of a triangle, preferably an equilateral triangle. The pressure plate is preferably supported in the center of this triangle on the actuating member. At least two of the application points for the actuating member are formed on one of the snap switches each. The third point of application is either stationary or, if a three-fold switching over possibility is required, it is formed on a third snap switch. If, proceeding on the basis of a state of rest, in which the membrane is not loaded with a pressure, the external pressure acting upon the membrane, and thus the external force acting upon the actuating member, gradually increases, and finally reaches an amount predetermined by the prestress on the first snap switch, then the first snap switch snaps, as corresponds to the essence of a snap switch, out of its original position abruptly into its final position. The actating member follows the first point of application, in that it swivels about a conceived axis, which is determined by the second and the third point of application. The point of the actuating member, which originally lay on the first point of application, moves along an arc with this swivelling of the actuating member. In general, for reasons of space, it is not possible to arrange the first snap switch in such a way that its point of application moves along the same arc when snapping over. Consequently, when the first snap switch snaps, over a relative shift takes place between the first point of application and the actuating member, in which case a frictional resistance must be overcome. This frictional resistance cannot be kept sufficiently constant in series production, and changes in the course of time due to the fact that the mostly small surfaces slipping over each other either become smoother, when the respective snap switch snaps over frequently, or they gradually become rougher by corrosion. At a given magnitude of the frictional force, the greater the relative shift between the point of application and the actuating member, the greater is the energy consumed by friction.
What has been stated in the foregoing in connection with the first snap switch applies analogously for the second snap switch when it snaps over, as well as for the third snap switch, if any. However, the greater the frictional energy consumed with snapping over of the individual snap switches due to relative shift between their point of application and the actuating member, the smaller is the accuracy with which a certain switch-over pressure can be predetermined for each snap switch by adjusting the spring prestress of the respective snap switch.